Matt Whiting
:

wrote:
Bertie the Bunyip wrote:

...
What th ewirter seems to be describing is an arrangement where two

opposing

pistons share a crankpin. I've seen twins like this and they vibrate

like

hell. A boxer (whihc most all flat opposed engines are) is a much

smoother

arrangement.



What is the distinguishing characteristic of a boxer, that opposing
cylinders fire simultaneously?


I think the main feature is that the cylinders go in and out in
opposition thus giving very good primary balance. You always have the
torsional vibration from firing and there is also secondary imbalance.

that's right. I have a BMW bike that's a twin boxer. there's two throws on
the crank and the pistons go in and out in oppostion to each other. AFAIK
all flat four airplane engines are the same, (though I'm sure there's some
ddball one out there somewhere) The ignition is alternating, though..
There have been some twins that have a single throw crank. It's as if
someone said "hey, that old single cylinder engine vibrates like hell, but
you know what'd make it vibrate even more? If we added another cylinder on
the other side!" These engines, while lighter than a boxer, have the added
disadvantage of supplying their power strokes at uneven points in the
rotation cycle. So you get two quick thumps and then a space in between..


Bertie